Photoelectric instrument for direct spectrochemical analysis by the internal standard method



Dec.-11, 1951 J. L. SAUNDERSON ETAL 7,8

PHOTOELECTRIC INSTRUMENT FOR DIRECT SPECTROCHEMICAL 7 ANALYSIS BY THEINTERNAL STANDARD METHOD Original Filed Feb. 27, 1946 5 Sheets-Sheet l nW Z M nm m e w r SJ 6 e 5 o -R w d .Rt RE I a P i L m i e a z W J V E -NMN kNmXQQQ W HW\NN I /WN m %N BER 1951 J. L. SAUNDERSON mm. ,3 5

PHOTOELECTRIC INSTRUMENT FOR DIRECT SPECTROCHEMICAL M ANALYSIS BY THEINTERNAL STANDARD METHOD Original Filed Feb'. 27, 1946 5 Sheets-Sheet 2Sp ec/ra/ /in@ Jason L. 80unans0n my 5 Vic/0r J Ca/a ecourf E aye/7e W.Peferson v.1. 1,, S AUNDERSON ET AL 2,577,815 PHOTOELECTRIC' INSTRUMENTFOR DIRECT SPECTROCHEMICAL Dec. 11, 1951 ANALYSISBY THE INTERNALSTANDARD METHOD Original Filed Feb. 27, 1946 5 Sheets-Sheet 3 X5035 WQBQ Vic/0r J Ca/a ecour/ Eqyene W Pe/erson Dec. 11, 1951 J. L.SAUN'DERSON ETAL 2,577,815

, PHOTOELECTRIC INSTRUMENT FOR DIRECT SPECTROCHEMICAL ANALYSIS BY THEINTERNAL STANDARD METHOD Original Filed Feb. 27, 1946 5 Sheets$heet 4ower 65 line 64 66 65 7'0 spark source To background ska/fer and/everJ/ng re/ay:

To mas/er 3/: u//er To .s/ror/ing re/ays To re Cara er mo/or re/oy Tomeasuring re/ay: 7'0 /e0 back re/ag 7'0 recor d'ng ,o ens From powerpack From re/rence Jasan L. 60unaers0n V/c/ar-J Ca/oeaour/ E 1.19am? MlPe/ergson Dec. 11, 1951 J. 1.. ,Js'AuNDzRsoN ET AL Y 2,577,815PHOTOELECTRIC INSTRUMENT FOR DIRECT. SPECTROCHEMICAL ANALYSISBY THEINTERNAL STKNDARD METHOD Original Filed Feb. 27, 4946 -5 Sheets-Sheet 5Q QG $6 $6 m6 N6 QNQWN Wmwm W /6 M yan i SQ w bm m 8 Q saw \v m me kw wm wmwk nmQiNQEQQEQQ \G mmG S V 1 m M Mam m Q9 Patented Dec. 11, 1951PHOTOELECTRIC INSTRUMENT FOB DI- RECT SPECTROCHEMICAL ANALYSIS BY THEINTERNAL STANDARD METHOD Jason L.' Saunderson, Victor J. Caldecourt, and

Eugene W. Peterson, Midland, Mich., assignors to The Dow .ChemicalCompany, Midland, Mich., a corporation of Delaware Original applicationFebruary 27, 1946, Serial No.

Divided and this application December 15, 1949, Serial No. 133,181

4 Claims.

This invention relates to a direct-reading or phototelectricspectrometer for use in spectrochemical analysis. It concernsimprovements both in the optical system and in the electrical measuringcircuits of such an instrument.

This application is a division of our co-pending application Serial No.650,676, filed February 27, 1946.

Quantitative spectrochemical analysis by the internal standard method,an exceedingly valuable tool for the metallurgical industries, hasheretofore been carried out almost entirely by photographic means, suchas those described, for example, in U. S. Patents 1,979,964 and2,043,053. This method, while of broad usefulness, is, for routineprocess control, disadvantageous in that a considerable period of time,at least several minutes and often much longer, elapses between theexposure of the photographic plate and its development andinterpretation.

It has been appreciated that a considerably greater rapidity of analysismight be realized if the spectral lines to be compared, instead of beingrecorded photographically, were focused on separate photocells, therelative outputs of which could then be measured electrically. However,the photoelectric spectrometers which have been proposed leave much tobe desired in the way of simplicity of operation, as well as inmattersof inherent accuracy and of stability in the presence ofvibration and ordinary environmental changes.

-It is therefore an object of the invention to provide an improveddirect-reading spectrometer which may be made fully automatic and whichis usually capable of giving complete analyses for as many elements asdesired in a period of less than one minute. Another object is toprovide an instrument of this character which may be operatedcontinuously over long periods by nontechnical personnel and yet exhibitan inherent accurancy equal to or exceeding that obtained with thespectrograph. A further object is to provide a spectrometer in which therelative accuracy of analysis for any given element is essentiallyconstant over wide ranges of concentration of that element.

Another object of the invention is to provide an instrument embodyingimproved electrical means for integrating and comparing the relativeoutputs of two or more photocells when each is exposed to light ofcontinuously varying intensity. Additional objects are to provideoptical means for compensating for background radiation which may bereceived by a photocell together with a desired spectral line andelectrical means for balancing out the dark current of the cell. Afurther obiect'is to provide an improved circuit for determining'theelectric charge imposed by a photocell on a condenser, such circuitincluding a highly stable direct-current electrontube amplifier.

Still other objects and advantages of the invention will be apparentfrom the description to follow, which is made with reference to theaccompanying drawings, in which Fig. 1 is a simplified diagramillustrating the inter-relation of the essential elements of theapparatus;

Fig. 2 is a horizontal section through the photocell compartment of thespectrometer, showing the arrangement of the optical elements;

Fig. 3 is a sketch illustrating the centering of a spectral line in anexit slit of the spectrometer, the line being shown enlarged withrespect to the width of the slit;

Fig. 4 is a similar sketch illustrating the positioning of a spectralline in an exit slit when a background compensating prism is used;

Fig. 5 is a sketch showing the details of another slit arrangement forcompensating for background spectral radiation;

Fig. 6 is a horizontal section through a photocell and its diffuser;

Fig. 7 is a diagram of the electrical measuring and recording circuitsof the apparatus;

Fig. 8 is a circuit diagram of the automatic tim ing system of thespectrometer;

Fig. 9 is a detailed circuit diagram of one of analysis of the PRINCIPLE1 In the apparatus of the invention, light produced by sparkingelectrodes of the sample to be analyzed is resolved by a spectrometerwhich focuses characteristics spectral lines of the'reference andunknown elements on sensitive photocells, causing the fiow of minutephotocurrents. The varying intensities of the lines during the sparkingperiod are integrated by storing the cor-- respondingly varyingphotocurrents in condensers. After the sparking period, these storedcharges are compared by allowing the condensers to discharge throughsimilar resistances and measuring the difference in the times requiredfor such discharges to take place. Electron-tube amplifiers follow thecourse of the condenser discharges and acuate relays which operate arecorder, the indications of which can be calibrated directly in termsof the analysis of the sample being sparked.

SIMPLIFIED DIAGRAM The essential elements of the new apparatus may beexplained with reference to the simplified diagram of Fig. l.

The material to be analyzed is formed into electrodes H which are causedto spark by connecting them across a high voltage source II. Light fromthis spark is focused by a lens it through an electrically-openednormally-closed master shutter 14 onto the entrance slit ll of aspectrometer it. The entering light falls on a concave grating H, whichforms images of the slit along the Rowland circle, the positions ofthese spectral lines being dependent on the wavelengths present in thelight. Exit slits II are located along the circle in positions to allowpassage of selected lines corresponding to the internal referenceelement and the element being analyzed for in the electrodes I I. Theseselected lines are focused by lenses is on the sensitive elements ofelectron multiplier photocells It, the operating potentials of which aresupplied by a hi h voltage source 2|.

During sparking of the electrodes, the spectral line falling on eachphotocell causes a 'corresponding fiow of current, which is stored in acondenser 22 connected in the circuit through a two-way switch 24.Later, the relative magnitudes of the charges thus accumulated aremeasured by connnecting each condenser across its individual dischargingresistor 23 by throwing the two-way switches 24. when these switches arethrown, the falling potentials of the condensers are followed byelectron-tube direct-current amplifiers 25 in the output circuits ofwhich are voltage-responsive relays 28 and 21 with their outputterminals in series. The relay 2! in the circuit corresponding to thespectral line of the unknown element is of the normally-closed type,while the relay 21 in the circuit of the reference element is normallyopen. Whenever operation of the amplifiers holds the contacts of bothrelays closed at the same time, the pen circuit ll of a moving-taperecorder 28 is energized by a source III, causing a mark to appear onthe tap In practice, it is necessary to choose the reference spectralline, or to design the circuits, so that during sparking the referencecondenser reaches a higher potential than the other condensers. Therelays and amplifiers are adjusted so that the relays are actuatedwhenever the potentials across the condensers exceed a predetermined lowvalue, conveniently about 1.0 volt.

In making an analysis, the switches 14 are set to connect the photocellsto their condensers, and the sample to be analyzed is sparked. Theresulting spectral lines falling on the photocells a cause photocurrentswhich charge the condensers 22 at rates proportional to the intensitiesof the incident lines. After the sparking period, the switches 24 arethrown simultaneously, whereupon the condensers I! begin to dischargethrough their resistors 23. As soon as the switches close, theamplifiers ll detect condenser voltages well over the predetermined lowvalue,

and actuate both relays 20 and 21, leaving the recorder circuit stillopen. As discharge of the condensers continues, the falling potential inthe condenser corresponding to the unknown element reaches thepredetermined low value first, whereupon the relay II is de-actuated andcloses. Current at once fiows in the recorder circuit II. making avisual record on the moving tape. When, after an interval of time, thefalling potential across the reference condenser also reaches thepredetermined value. the relay 21 is deactuated, and the recordercircuit is again broken. As a result of this sequence, the length of themark on the moving recorder tape is a direct indication of thedifference in times of discharge of the reference and the unknowncondensers. This difference is a function of the concentration of theunknown element in the sample, as will be evident from the followingconsiderations.

GENERAL CONSIDERATIONS Theory where E0 is the initial voltage. E is thevoltage after time t, and e is the base of natural logarithms.Considering two condensers of capacities Cu and Cr, having initialvoltages Eu and E: and discharging through resistances Ru and Br. thetimes in and tr required for the condensers to reach a predetermined lowvoltage E. are. from Equation 1:

(a) ta=Raca 108s Ea/Es (3) tr=Rrcr 108s lr/E'a If the resistances andcapacitances are chosen so that their products are equal to someselected value RC. that is. if

(4) RaCa=R1Cr=RC then, by combining Equations 2, 3. and 4, and takingtr-ta=At, it follows that (5) At=RC 108s Er/Ea But the initial voltagesEr and Eu on the condensers are proportional to the total charges, andhence also proportional to the integrated intensities Ir and In of thespectral lines. whence. sub

stituting in Equation 5, there appears:

'(6) At=RC log bla-i-constant From spectrochemicai theory, it is knownthat the ratio of the intensities of the spectral lines of the referenceand unknown elements is approximately proportional to the ratio of theconcentrations x. and x of these elements. Since the reference elementis essentially of constant concentration, it follows from Equation 8that Aim-RC log. xe+constant gamers From these considerations, then, itis seen that a plot oi At, the difference in times or discharge of thereference and unknown condensers, against the logarithm of Xu, theconcentration of the unknown element, is essentially linear.

In the apparatus of Fig. 1, this time diflerence. At, is, as explained,read directly from the length of the mark on the recorder tape. Inmaking analyses, then, the apparatus is first calibrated by determiningthe length of the recorder mark for several samples in which the elementbeing analyzed for is of known concentration. Then a line drawn througha plot of these lengths against the logarithms of the concentrations ofthe element gives the desired calibration chart. From this, analyses ofunknown samples can be run by simply sparking the sample according toSeveral important advantages arise from the fact that, in the apparatusof the invention, the quantity actually measured, viz. the dififerencein discharge times of two condensers, varies logarithmically with theconcentrations to be determined. In the first place, the spectrochemicalmethod in general tends to have an essentially constant relativeaccuracy for the quantitative determination of an element over wideranges of concentration. For instance, the absolute error in determiningan element'at per cent concentration is roughly about ten times as greatas the absolute error at 1.0 per cent concentration. The logarithmiccalibration curves of the present apparatus are similar in this respect,in that they may be read with constant relative accuracy over the wholescale. A second advantage, apparent from Equation 7 above, is that theslope of the logarithmic calibration curve is independent of changes inapparatus conditions, since all proportionality factors fall into theconstant in the equation. Thus, a change in the sensitivity of aphotocell circuit, or dirt in the optical path, will only shift thecalibration curve parallel to itself without altering its slope. Inconsequence, if a change in the apparatus occurs, it is necessary, inorder to recalibrate, to run only a single standard sample, to give a Atvalue which, with the constant'known slope, will give the complete newcalibration curve.

An additional advantage of the apparatus of the invention is that,although the instantaneous intensities of the spectral lines, and hencealso the instantaneous photocurrents, may vary considerably due toflickering of the spark, the condensers, in storing these currents,effect a complete integration over the time of sparking. As a result,when the charges on the condensers are measured, there is obtained anintegrated relative intensity value for the two spectral lines which ispractically independent of variations in both the total intensity of thespark and the time of sparking. Since this integration is quite similarto that efiected by the photographic plate of a spectrograph, the methodof the invention permits use of virtually all the established techniquesor the internal standard method so well known in spectrographicanalysis.

Background compensation there is, at every wavelength, a certain amountof background radiation. In the apparatus of the invention, some part ofthis background enters the photocell together with the line on which itis focused. As long as the intensity of the line is high relative tothat or the background, the latter is of little consequence. However,when the cell is focused on a line of weak intensity, as, for instance,when analyzing for an element present in low concentration, thebackground may bea substantial part of the total light incident on thecell. While this circumstance does not render analysis impossible, sincethe apparatus is calibrated empirically, the relative accuracy ofanalysis will be reduced, largely because the intensity of thebackground tends to vary somewhat from sample to sample.

In the invention, this effect of background radiation'may be largely orwholly overcome by an optical system including a shutter which permitsrepeatedly exposing the photocell first to the desired line togetherwith its background and then to a selected portion of the backgroundalone. At the same time, operation of the electrical circuits issynchronized with the shutter so that the condenser associated with aphotocell, while being charged by the cell each time the latter isexposed to the line and background, is also allowed to discharge throughthe cell each time the latter is exposed to the background only. Bymaking the times of charging and discharging equal, the additive effectof the photocurrent due to the background during charging is nullifiedby its subtractive efiect during discharging. At the end of the sparkingperiod, then, the condenser charge contains little or no componentattributable to the background, and is a function only of the intensityof the desired line.

Dark current compensation The method of background compensation justdiscussed has the further effect of also compen-' eating for the darkcurrent of the photocells, thereby rendering frequent adjustment of thecell circuits unnecessary. However, an alternative method of darkcurrent compensation may also be desirable, especially for use in thoseinstances where background compensation is not required. Such darkcurrent compensation may be accomplished by connecting a high resistanceacross the photocell at the time it is charging its condenser, andarranging this circuit so that the current flowing through theresistance exactly equals the dark current. Balancing means for makingthis adjustment are provided and will be described later.

Direct current amplifier In the present spectrometer, the direct-currentamplifiers which follow the discharge of the photocell condensers musthave a high gain in order properly to actuate the relays in their outputcircuits. On the other hand. they must also 7 be accurate and stable,preferably with so little drift that they require adjustment only onceor twiceaday. Thisisachieved.accordingtothe invention, by means oftwo-stage direct-current amplifiers which are provided with feed-backnetworks to render them highly degenerative and hence stable. .Theamplifiers are normally maintained in the degenerative state. However,at the moment of discharging the photocell condensers. the feed-backnetworks are temporarily broken, converting the amplifiers to asensitive orhigh gain condition. After the discharge, the feedbackcircuits are again closed, returning the ampliher to a stable state inwhich any tendency toward drift is compensated.

DESCRIPTION OF APPARATUS are not shown. Adequate descriptions of theseelements, as well as methods of preparing the electrodes, are given inJ. Opt. Soc. Amer. 34, 104-5 and 116-9 (1944).

Optical details ,The spectrometer housing II and the grating 11 (Fig. 1)are of known type. It is important that the spectrometer be rigid andsolidly mounted, and desirable that the housing be maintained at aconstant internal temperature, conveniently 90 F., as by a thermostatedcirculating air heater.

As shown particularly in Fig. 2, the spectrometer exit slits it areformed in thin metal plates II which are adjustably mounted over slots32 in the spectrometer housing It along its focal curve II in positionsto pass the desired spectral lines. In general, the width of an exitslit should be about two times that of the entrance slit for thoselines, such as the reference line, on which no background correction isnecessary. and about four times on lines on which a correction is to bemade.

. If no background correction is needed, as in the case of the line ofthe reference element, the line is approximately centered in the exitslit ll (Fig. 3). However, when a background correction is made, as inthe case of the line of an unknown element, the line is directed throughone side of the slit, allowing the background radiation to pass throughthe other side (Fig. 4). Inthis case, a small prism 3| is centered justbehind the slit so as to split the light into two beams, one of whichincludes the desired line and the other contains only the background.Each of these beams enters the lens I9 and is focused on the photocell20 (Fig. 2). The purpose of the prism is to separate the two beamssufficiently to allow satisfactory operation of the backgroundcompensation mechanism.

This compensation is effected by means of a shutter interposed betweenthe exit slits and the lenses and consisting of a curved metal striphaving slits 36 cut in it in positions corresponding to the exit slitsand being somewhat wider than the latter. This shutter is mounted toreciprocate in a fixed guide 31, and is moved 8backandforthbyopposedsolenoidsflwhich are attached to either end.Movement of the shutter is limited by stops it one of which is adjustedso that at the limit of shutter travel in one direction the shutter isin a position to allow free passage of the spectral lines from the exitslits to the lenses and to block the background beam (as shown in Fig.2). The other limit of travel is such that the shutter is in a positionin which, in the case of lines requiring no background compensation, itcuts oil all light to the photocell, and in the case of lines requiringcorrection, it cuts off the spectral line but allows free passage of thebackground beam. The shutter is reciprocated by alternately activatingthe solenoids in synchronism with the photocell condenser reversingcircuits, as will be later described.

The exit slits, prisms, shutter mechanism, lenses, and photocells areall mounted rigi ly on the frame of the spectrometer by means not shown,and are enclosed in a light-tight case I. The lenses and prisms are ofquartz or other material transparent to ultraviolet light.

An alternative method of background correction, useful when theimmediate background of a selected line contains interfering lines, isshown in Fig. 5. In this case, a second exit slit 4| is located near theslit is for the desired line, but in a non-interfering spectral region,and the shutter 35 is adjusted to cut off beams from the two slitsalternately. Both beams are, of course, focused on the same photocell. 1

While Figs. 1 and 2 illustrate a spectrometer with onh! two exit slitsand photocells, one for the reference element and one for the unknown,it will be appreciated that any desired number of exit slits andphotocells may be arranged along the focal curve at positionscorresponding to various spectral lines to permit determination ofseveral unknown elements at once, using the same; or more than one,reference spectral line. In such cases, certain of the beams passingthrough closely adjacent slits may, if desired, be deflected by mirrorsbefore entering their corresponding lenses and photocells, thus avoidingcrowding together photocells directly behind the focal curve.

The considerations involved in selecting appropriate spectral lines forany given analysis are similar to those in spectrographic work, exceptthat greater latitude in choosing lines is possible. (See discussion ofSaunderson et al. in J. Opt. Soc. Amer. 35, 681 (1945). a paper whichalso describes the present invention.)

Photocells While any of a number of types of photocells may be employedin the apparatus, optimum results are obtained with phote-emissivetubes, particularly electron multiplier photetubes, such as the 931A,which combine extreme sensitivity with high amplification. These latterphototubes, however, present'a special problem because of the greatvariationin output occasioned by slight changes in the position ofincidence of the spectral line.

Thus, as will be appreciated, from Fig. 1, while the positions of thespectral lines of the elements are fixed, the light beams, after passingthe exitslits, move with every motion or fiicker of the spark betweenthe electrodes Ii. The lenses it, in focusing the beams on thephotocells III, eliminate the eflfects of motion in the light sourceexcept for aberrations resulting from using difquartz plate 42 mounted ashort distance in front of the envelope 43 oi the photocell, the spacebetween the plate and the envelope being surrounded by a light shield44. With this construction, critical positioning 9! the phototubes isnot required.

Measuring circuit I The basic electrical measuring circuits of theapparatus may be explained with reference to Fig. 7. For simplicity, thediagram shows only three multiplier phototubes and measuring circuits,for determining the relative intensities of two unknown spectral linesrelative to the same internal standard line. In Fig. 7, the timingcircuits for actuating the various relays are not shown, and theelectron-tube amplifiers are indicated in simplified form. The circuitis shown in its normal position before an analysis is run.

The operating potential for all the electronmultiplier tubes is suppliedby a 1000-volt regulated power supply as constructed essentiallyaccording to the Miller design (Electronics, November, 1041, p. 29). The250-volt amplifier power pack 56 is of a similar type.

The dark-current of each phctotube 20 is compensated by a leak circuitin. which the photocell anode ill ,is connected through a high re--sistance as (1000 megohms) to a potentiometer it (100 volts), oneterminal of which is grounded.

During the sample sparking period, the output of each photocell, exceptfor the small darkcurrent compensation. passes in series through amanual test switch 50 and the normally-closed contacts of a relay to itsstorage condenser 22, the return circuit being through the normallyclosed contact of a relay 54. Actuation of the relay 54 interposes astandard cell 52 in the return circuit. The polarity of the condensermay be changed by a reversing relay 52, and the condenser is shorted,except during measurements, by another relay 53.

At the end of the sparking period, each condenser may be isolated iromits photocell and connected directly across its discharging resistor 23by operation of the relay 5|. As this operates, each condenser isconnected through a bias cell 55a to the input grid 55 of thecorresponding amplifier 25, which in turn actuates a correspondingrecorder relay 26 or 21.

Each amplifier 25, which will be more fully described later, comprisestwo stages and is stabilized by a feedback circuit 56 which may bebroken by a relay 51. The magnitude of the amplifier output is indicatedvisually by an electron-ray tube 58, the ray-control grid of which isconnected in the plate circuit of the'second amplifier stage.

The recorder relays 28 and 21 in the amplifier outputs control therecorder pen circuits 28, as

already explained with reference toFig. l. The

recorder, which is conveniently oi the movingtape type described inPatent 2,251,742, is driven by a self-starting synchronous motor 59, the

they have been discharged rapidly, condensers 10 using polystyrene asthe dielectric being especially suitable (J. Opt. Soc. Amer. 35, 690(1945). The electrical magnitudes of thecapacitance of each condenser(0.1 to 1.0 microtarad) and of the resistance 01' each dischargingresistor (5 to 50 megohms) are chosen so that their mathematical productis of the order of several seconds and is substantially equal for allcondenserresistor pairs in the apparatus, in order to satisfy themathematical requirements 01 the system, as in Equations 4 and 5 above.

Operation of circuit As long as the relays are in the positions shown inFig. 7, the input grids 55 of the amplifiers 25 are all maintained at afixed potential below ground potential determined by the bias cells 55a.With this condition obtaining, the operator first adjusts eachamplifier-relay circuit so that each indicator tube 58 or magic eye isjust "closed," and so that each recorder relay 26 or 21, though still inthe normal position, will be activated whenever the correspondingamplifier input-grid potential is depressed below the aforesaid biaspotential.

' With the amplifiers thus normalized, and with the photocells 20receiving no light, dark current compensation is made. Each test switch50 is thrown, connecting each photocell directly to its amplifier inputgrid. At the same time, the feedback relay switches 51 are-opened toplace the amplifiers in the high-gain state. ,Each voltage-divider 49 isthen adjusted 'until the corresponding indicator tube 58 is againclosed." The entire dark current of each photocell is thus balanced out,and fiows through its leak resistance 188. The switches 50 and 5'! arereturned to normal positions, and the measuring circuits are ready foruse.

In making an analysis with the spectrometer utilizing the measuringcircuit of Fig. 7, the sample to be analyzed is first sparked for a fewseconds, after which the exposure period is initiated by a timing systemlater to be described which actuates the master shutter I4 (Fig. 1) andthe condenser shorting relays 53 (Fig. '7) at the same instant. Thelight 01' the spark then enters the spectrometer and the resultingspectral lines fall on the photocells 20, causing photocurrents to flow;the condensers 22, simultaneously unshorted, store these photocurrentsthroughout the sparking period. As sparking proceeds, the solenoids 38for reciprocating the background compensation shutter (Fig. 2) and thecondenser reversing relays 52 (Fig. 7) are operated in synchronism.Thus, each condenser is alternately charged with photocurrentcorresponding to the intensity of the spectral line plus background anddischarged for an equal period with photocurrent corresponding to theintensity of the background alone.

When the exposure period has ended, the timing system closes the mastershutter ll, cutting oil. all light to the photocells. It simultaneouslyplaces the background shutter solenoids 38 and the condenser reversingrelays 52 in normal positions, and closes the recorder motor relay 60,setting the recorder roll Si in motion. The condenser measuring relays5| and 5| and the feedback circuit relay 5! are then actuatedsimultaneously, connecting the condensers 22 across their dischargingresistors 23, isolating the amplifier input grids 55 from ground,connecting the standard cell 62 in circuit, and converting theamplifiers to a high-gain condition. Assuming that the potentials on thecharged condensers all 11 exceed in magnitude that of the standard cell.the amplifier input grids it instantly detect potentials below the biaspotential, and the amplifiers actuate the recorder relays and 21. The

recorder pen circuits is for the unknown elements remain open, but thecircuit for the reference element, involving the relay 21, is at onceclosed, so that recording begins on this one pen. Then, when thedecreasing potentials on the condensers of the unknown elements reachvoltages equal in magnitude to that of the standard cell. i. e. when thegrids ll reach bias potential,

the relays It are released, and the remaining recorder pen circuitsclose and cause marking of the recorder tape. As soon as the decreasingpotential on the reference element condenser also M l s that of thestandard cell, its amplifier deactuates the relay 21, and the pencircuits are again broken, so that all recording stops. The condenserdischarge relays II and ll, the condenser shorting relay the amplifierfeedback relay I1, and the motor relay it are then all released, placingthe circuits in the normal or inactive condition, as in Fig. 7, andreturning the amplifiers to the degenerative or self-stabilizing state.The entire apparatus is then ready for another analysis.

Timing system The details of the timing mechanism for initiating theoperating impulses for the various relays in the measuring and sparkcircuits of the direct-reading spectrometer are shown in Fig. 8. Thecontrolling elements of the mechanism are two timers and a series ofcam-operated switches, togetherwith several relays. The cams rotatecontinuously when the spectrometer is in use, while the other elementsfunction only in a predetermined sequence each time the "start" switchis operated to make an analysis.

In the circuit of Fig. 8, which is shown in its normal or inactivecondition, the current is drawn from a line source 0, the return circuitbein in all cases through ground. The current first flows through anormally closed cut-out relay M to the "start" switch ,and from there tothe coil of a self-locking normally-open starting relay This startingrelay, when closed, supplies current through a line I] to operate thespectrometer spark source l2, and also to actuate a first timer switch6.. This latter is set to allow the elapse of several seconds for theelectrode spark to stabilize itself before starting the condensercharging cycle.

When the timer switch ll closes to start the spark exposure period,potential is supplied from the line It to the start cam switch as, thecam of which is mounted on a shaft It turned continuously by a gearedsynchronous motor II at a slow rate, conveniently once very two seconds.As soon as this cam switch it closes, the control impulse fiows througha lead 12 to the coil of a multi-contact self-locking starting relay l3.

- When this starting relay is actuated. power flows 1! supply circuit IIand the lead It to the relay I. operating the spectrometer recordermotor ll (Fig. 7). The entire system of Fig. 8 then remains in thiscondition until'the exposure period is ended by operation of the secondtimer ll.

As soon as this second timer switch I! closes. current fiows' from apower supply lead I2- lIa-lib to energize a quick-closing delayedreleasecontrol relay 88, which in turn actuates the coil of a multi-contactstopping relay ll. When this latter relay II is energized, it connectsthe power supply lead ll-OIc-"c to the coil of the control relay 0, thuslocking both the control and the stopping relays in the actuatedposition. This same actuation of the stopping relay it also connects thepower line I to the power supply circuit ll for the recorder motor.which circuit is connected by a Jumper it to the lead II for thecondenser shorting relays. thus preventing the latter from beingdeactuated. Likewise, actuation of the stopping relay ll closes acircuit ll connecting the stop cam switch IT with the ground side of thecoil of the starter relay II. Thus, as soon as the stop cam rotates toclose the switch 01, the latter shorts out the coil of the starter relayII, deactuating the latter. The starter relay II at once drops out,breaking the power supply lead 14 to the master shutter It, thusterminating the exposure period, and also allowing the timer It toreset.

This dropping out also connects the power supply circuit 00 to the lead0| to the recorder motor relay (Fig. 7),, starting the motor 59.

This same current supplied to the recorder lead It also flows through alead II to the coil of the cut-out relay l4, actuating it and breakingthe power supply to the locked starting relay N. This latter then opens,shutting oil the power supply to the spark source I! and the first timerswitch ll, allowing the latter to reset.

The current in the recorder motor lead It is also transferred by thelead 88 and a second lead I! to the measuring cam switch so, which, whenit rotates into closed position, energizes the coil of a multi-contactself-locking measuring relay 9|. This latter, on closing, connects powerfrom the source lead I! to leads 92 for the condenser measuring relaysII and II, and the amplifier feedback relays I! (Fig. 7), thus startingthe measuring cycle of the instrument. This closing of the measuringrelay II also breaks the connection between the power lead I! and thelead 82a, but it is simultaneously re-established by the closing ofauxiliary contacts on the reference amplifier recording relay 21, whichis actuated by its amplifier at the same instant that the measuringcycle begins, as already explained with reference to Fig. 7.

When the measuring cycle is completed, the reference amplifier releasesits relay 21, thus breaking the power supply to the power lead Ila-82cand de-energizing the coil of the control relay II. Thisdelayed-release-relay is set to remain closed a few seconds to allow therecorder motor time to run out extra tape, after which the relay opens.When it opens, the stopping relay I4 is thus deactuated, and shuts of!the power to the recorder motor leads I0 and II. stopping the motor andde-energizing both the cut-off relay N and the measuring relay OI,returning all the circuits to the normal state ready 7 for the nextanalysis.

As shown in Fig. 8, the cams of the starting switch it and the stoppingswitch I! provide only momentary closing of their respective circuits,and are set so that they'operate simultaneously. The reversing camswitches 16 close their circuits alternately, each circuit remainingclosed for one half revolution of the camshaft. The particular reversingcam which holds the background shutter in position to transmit thespectral lines is timed to close simultaneously with the start and thestop cams. The cam of the measuring switch 90, which likewise closes itscircuit only momentarily, is set to lag the stop cam by an appreciableinterval, such as onefourth of a revolution, to allow the recorder motortime to reach speed before the measuring cycle begins.

The circuits of Fig. 8 render operation of the spectrometer entirelyautomatic. .Thus, the operator, after inserting the electrodes to beanalyzed into their holders, merely pushes the start button 65. Thespark' across the electrodes is at once turned on. Then, after a warmupinterval determined by the timer 68, the

\ master shutter on the spectrometer inlet and the condenser chargingcircuits operate as soon as the starting cam 69 permits. The backgroundshutter remains in position to pass the spectral lines for a full seconduntil moved by the reversing cam, after which the shutter reciprocatesat one-second intervals. When the second timer l9 indicates that thecondenser charging period should end, the stop cam 81 closes the mastershutter and turns off the spark source, both at the end of the nextbackground exposure of the shutter, this proper timing to insure anequal number of line and background" exposures being maintained by theaction of the stop cam 8?; independent of the setting of the secondtimer. As soon as the charging period has ended, the recorder motorstarts up. After a halfsecond interval fixed by the measuring cam 90,during which the motor reaches its speed, the measuring cycle isinitiated, control of all circuits is transferred to the referenceamplifier I relay 2?, and the making of a record on the recorder tapebegins. When all condensers are discharged to the predetermined lowpotential, the recorder motor continues to run out tape for a briefinterval set by the delayed-release relay 83, to give enough space topermit the tape to be torn ofi easily, after which all circuits returnto their normal state ready for another analysis. The entire operation,which requires thirty to forty seconds to complete an analytical recordready to be interpreted, is thus fully automatic. Errors due to thehuman element are practically eliminated, and a permanent record,available for checking at any time, is made.

Amplifier A circuit diagram of the direct-current amplifier 25 used infollowing the falling potentials on the photocell condensers of thespectrometer is given in Fig. 9.

As shown, the first stage of the amplifier consists of a high gainpentode 93 with low screen and plate voltages, e. g. a 6J7 or 606, sothat the input resistance will be hi h. This first stage is 81, and athird stage or vpentode 98, e. g. a 6J7, the plate of which is coupledto the second grid of the first stage. The feedback circuit may bebroken by the feedback relay switch 51. The terminal of the switch 51connected through the battery 91 to the control grid of the third stageis also connected to a condenser 99, conveniently .of about 1.0microfarad capacity, the other side of the condenser being connected tothe cathode of the third stage.

As long as the feedback circuit is closed, the amplifier is in a lowgain, highly stable state and exhibits little drift. At the same time,the condenser 99 acquires a charge consistent with the balance in thesystem. Then, when the feedback circuit is opened by the relay 5'! inresponse to the timing system of the spectrometer, the amplifier isconverted to a high gain condition for the measuring period. However,during this period, the condenser 99 tends to retain its charge.preventing the third stage grid potential from changing appreciablyduring the high-gain period. Thus, when the relay 51 is again closed atthe end of the measuring period, the amplifier is returned to theoriginal stable condition. The amplifier of Fig. 9, by virtue of theaction of the condenser 99 and the feedback circuit, remains balancedalmost indefinitely, and requires adjustment of the voltage-divider 96perhaps only once aweek while in continuous use.

INTERPRETATION OF RESULTS Fig. 10 illustrates a typical length of therecorder tape of the direct-reading spectrometer direct-coupled to thesecond stage 94, comprising 1 two triodes in one envelope, e. g, a6SN'I, operated at conventional voltages and connected to stabilize theplate current. The output of this stage is supplied to the recording penrelay 26, as previously described. The amplifier is rendered highlystable by a feed-back circuit comprising neon glow lamps 95, avoltage-divider 96, battery containing the record of a single analysisof a magnesium-base alloy. The particular record shown was obtained witha spectrometer utilizing eight photocells and gives the analysis of eachof seven elements relative to magnesium as the internal referencestandard.

In making 'the analysis, the recorder was set to move the tape at aconvenient speed, in this case 1.0 inch per second, the duration of themeasuring period being about 12 seconds. The recorded line of eachelement on he tape is made by a pen associated with the photocell,amplifier, and relay corresponding to that element, as already set forthin detail. When the spectrometer reaches its measuring cycle, therecorder first starts the tape moving (from right to left, Fig. 10).Then, as soon as discharge of all condensers begins, the line formagnesium, the reference element, starts to appear. When the potentialof the discharging reference condenser reaches that of the standardcell, drawing of all lines ceases and the recorder then stops. Thelength of the magnesium recorded line thus is a measure of theintegrated intensity of the magnesium reference spectral line. Thelengths of the other recorder lines are measures of the intensities ofthe spectral lines of the unknown elements relative to magnesium, andhence measures of the concentrations of those elements, as explainedpreviously.

Fig. 11 is a typical calibration curve forthe unknown element aluminum.The curve was obtained empirically by determining the lengths of therecorder lines observed in sparking electrodes of several differentmagnesium alloys of known aluminum contents.

For convenience in reading the recorder lines. this calibration curve(Fig. 11) may be projected onto a straight scale (Fig. 12) which ismarked 01! to correspond to the aluminum contents as given by thecalibration curve. Then, to observe armors the per cent aluminum. asshown by the recorder tape, it is necessary merely to place thisprolected calibration scale beside the aluminum re-' corded line, withthe zero point of the scale opposite the end of the recorded line, andto read oi! the per cent aluminum from the scale at a point opposite theother end of the recorder line. Similar calibration scales for the otherelements shown on the recorder tape may be prepared in like manner, andare used in the same way.

In the event that the calibration curve of an element shifts parallel toitself, due to changes within the optical or electrical-systems of theinstrument, it is necessary merely to change the position of the zeropoint of the projected scale a compensating amount, the magnitude ofwhich may be readily determined byanalyzing a single sample of knownanalysis. In actual practice, a known sample is run every few hours, andthe zero points on the calibration scales of all elements correctedaccordingly. In general, the correction thus observed remain exceedinglysmall,

even over periods of many weeks.

It will be appreciated that the foregoing specification is descriptive,rather than strictly limitatlve, of the present invention, and thatnumerous variations of the detailed constructions shown are possiblewithout departing from the spirit of the invention, as defined in theclaims.

In this specification and claims, the term unknown elementmeans anelement the concentration of which is to be determined in the samplebeing analyzed and the term "reference element" means an element ofknown concentration in the sample which is being used as an internalstandard for the analysis.

What is claimed is:

1. In a direct-reading spectrometer having an exit. slit for a selectedspectral line, an electron multiplier photocell for observing theintensity of the line during an exposure period, and'a condenserconnected to the output of the cell: a prism mounted at the exit slit inposition to divide light passing therethrough into two beams, one beamconsisting of the spectral line together with adjacent spectralbackground and the other consisting of background only, a lens forfocusing both beams on the photocell, a shutter between the prism andthe photocell provided with means for repeatedly alternating itsposition at brief equal intervals to cut 01! first one beam and then theother, switching means for repeatedly reversing the polarity of thecondenser relative to the photocell in synchronlsm with the alternationsof the shutter, the polarities being such that the condenser is chargedby the photocell when the shutter is in position to admit the beamcontaining the spectral line, and means operative after the exposureperiod for indicating the charge on the condenser.

2. A direct-reading spectrometer for analysis by the internal standardmethod comprising, in combination with a spark source and aspectrometer: photocells for observing the intensities of the spectrallines of unknown and reference elements; individual condensers forstoring the outputs of the photocells during an exposure period;

at brief equal intervals to cut off first one of the beams to each celland then the other; reversing a master shutter for initiating andterminating the exposure of the photocells to the spectral lines;optical means for focusing on each cell switches for repeatedlyreversing the polarity of each condenser relative to its photocell insynchronism with the alternations of the compensating shutter, thepolarities being such that the condenser is charged by the photocellwhen the shutterisinpositiontoadmitthebeam containing the spectral line;and means operative after the exposure for comparing the charges storedin the condensers.

3. In a direct-reading spectrometer comprising a photocell for observingthe intensity of a selected spectral line during an exposure period. anda condenser connecwd to the output of the cell, the combinationtherewith of optical means for focusing on the cell two separate beams,one beam consisting of the spectral line together with its adjacentspectral background and the other consisting of background only, shuttermeans for alternately interrupting the two beams at brief equalintervals during the exposure period, switching means for ,irepeatedlyreversing the polarity of the condenser relative to the photocell insynchronism with the alternations of the shutter. and means operativeafter the exposure period for indicating the charge on the condenser.

4. In a direct-reading spectrometer comprising a photocell for observingthe intensity of a spectral line during an exposure period, and acondenser connected to the output of the cell, the combination therewithof alternating shutter means for repeatedly interrupting andre-establishing at brief intervals of equal length during the exposureperiod the optical path by which the cell observes the spectral line.switching means for repeatedly reversing the polarity of the condenserrelative to the photocell in synchronism with the said alternations ofthe shutter means. and means operative after the exposure period forindicating the charge on the condenser.

JASON L. BAUNDERSON. VICTOR J. CALDECOURT. EUGENE W. PETERSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,956,590 Pressler May 1, 19342,411,741 Michaelson Nov. 26, 1946 2,436,104 Fisher et al Feb. 17, 1948FOREIGN PATENTS Number Country Date 144,986 Austria Mar. 25, 1936 OTIERREFERENCES Publication-Long et a1.: Testing Shutter Speeds with thePhoto-Electric Cell, page 423 of the Photographic Journal, London,August 1934. (Copy in U. 8. Patent Ofiice Library.)

